The present invention relates to a process for manufacturing a dielectric member or more particularly, to a process for manufacturing a dielectric member from aluminum or aluminum alloy (referred to as aluminum hereinbelow) which includes the steps of subjecting the aluminum to anodic oxidation or anodizing to form thereon an oxide layer superior in electrical insulation and charging characteristics, and subsequently subjecting said oxide layer to surface treatment for further improvement of the electrical insulation and smoothness.
Commonly, for anodic oxidation or anodizing of aluminum, the anodizing processes utilizing sulfuric acid or oxalic acid as electrolyte have been most widely adopted. The conventional anodizing processes as described above, however, have such disadvantages that, since the electrolyte employed therein is in the form of aqueous solution, electrical insulation of the anodized or oxide layer formed thereby is low in volume resistivity, only approximately 10.sup.9 .OMEGA..multidot.cm, and thus the known processes as described above can not be applied to materials for which still higher electrical insulation is required.
On the other hand, various anodic oxidation processes employing electrolytes in the form of non-aqueous solutions have also been conventionally proposed, for example, in Japanese Patent Publication Tokkosho 39/18636 in which formamide and boric acid are employed as electroyte for simultaneously carrying out coloring through one process of anodic oxidation. The anodized or oxide layer obtained by the known process as described above is slightly better as electrical insulation than that available by the earlier described processes utilizing the electrolyte in the form of an aqueous solution, but still has insufficient electrical insulation in the region of approximately 10.sup.10 to 10.sup.11 .OMEGA..multidot.cm, thus not being suitable for cases where electrical insulation higher than 10.sup.12 .OMEGA..multidot.cm is required.
Furthermore, since the oxide layers formed by the known methods as described above are porous, they are active immediately after the formation thereof, and for rendering these oxide layers stable, pore sealing or filling treatment is normally effected with the use of pressurized steam or boiling water. Although inlets to the pores of the porous oxide layer are completely closed by the formation of hydroxides through the pore filling treatment, smoothness on the surface of the oxide layer is hardly improved. Moreover, the pore filling treatment with the use of the boiling water and the like has such a serious disadvantage that the insulating property of the oxide layer is deteriorated, with marked reduction of charged surface potential to be imparted to the surface of the oxide layer.
Consequently, the dielectric members obtained by the conventional anodic oxidation or anodizing processes have such disadvantages that they are poor as electrical insulation, thus not being suitable for uses in which high electrical insulation is required, for example, in electrostatic type printers, electrophotographic copying apparatuses, etc., and their smoothness and electrical charging characteristics are not improved.